The present invention relates to cellular radio communications in general, and more specifically, to a feed-forward multicarrier power amplifier architecture and to a method of and apparatus for spatially filtering unwanted intermodulation products using A feed-forward multi-carrier power amplifier in an array antenna architecture.
In noise limited radio communication systems, the ability of a radio receiver to receive a radio signal broadcast by a distant transmitter is limited by the amount of thermal noise present in the radio channel and the noise introduced by the receiver itself. In cellular radio communication systems, the ability of a mobile station to receive signals transmitted by a base station is limited not so much by the thermal noise in the radio channel, but rather by the amount of interference present in the radio channel. Interference arises from several sources: so-called co-channel interference arises from radio communications between base stations and mobile stations in adjacent or distant cells which occur on the same frequency band; so-called adjacent-channel interference arises from radio communications between base stations and mobile stations in the same cell or from adjacent cells operating on adjacent frequency bands. Imperfect filtering in the receivers and transmitters allows some of the radio frequency energy in one band to spill-over to and interfere with other radio frequency bands. Another potentially significant source of interference arises from intermodulation products generated by amplitude modulation to amplitude modulation (AM to AM) and amplitude modulation to phase modulation (AM to PM) conversion in the base station and mobile station amplifiers. This problem is usually most severe in the downlink (i.e., signals transmitted by the base station) since the base station often simultaneously broadcasts multiple carrier frequencies which can mix with one another in the amplifiers.
In power amplifiers, a trade-off is made between DC to RF power conversion efficiency and the level of intermodulation products generated by the amplifier. Thus, good DC to RF power conversion efficiency and high spectral purity can be contradictory requirements. The choice of amplifier is therefore significant in the design of the cellular base station architecture.
To date, there have been several base station architectures identified. Most commonly, base stations use a single carrier power amplifier (SCPA) with a frequency selective combiner. This architecture offers about 6-7% overall DC to RF power conversion efficiency due to the insertion losses encountered in the accompanying frequency combiner. The frequency combiner is also large and has "static" frequency selectivity which may need to be manually tuned during the base station installation.
Another common choice of architecture employs a multi-carrier power amplifier (MCPA). Unfortunately, MCPAs should generally be highly linear to avoid generating intermodulation products arising from the mixing of the different modulated carrier frequencies within the amplifier. Therefore, even though no frequency combiner is required, this solution only offers an overall DC to RF power conversion efficiency of about 4-6%. Although comparable to the above mentioned SCPA/frequency combiner solution, the MCPA typically has much lower robustness and reliability. A high power MCPA is also a complex technology, i.e., not easy to master in production.
Finally, an SCPA can be used with antenna combiners to give an overall DC to RF power conversion efficiency of approximately 22%, which is achieved at the expense of mast mounted power amplifiers and a surface area more or less proportional to the number of carriers. The robustness of this design can be improved by distributing small power amplifiers in an array, using spatial combination of a number of antenna elements instead of one central power amplifier per antenna.
Use of an MCPA and array antenna combining would be an attractive solution if the overall DC to RF power conversion efficiency of this solution could be improved. Current MCPA designs do not, however, promise to produce the needed improvements in efficiency.
The intermodulation introduced by MCPAs can be reduced by using one of two methods: feed-forward cancellation amplification, or linear amplification with non-linear components (LINC). LINC amplification is quite complex and is currently completely unsuitable for low-cost, mass produced amplifiers.
A block diagram of a conventional feed-forward cancellation amplifier is illustrated in FIG. 1A. In FIG. 1A, an RF input signal, whose spectrum is illustrated in FIG. 1B, is applied to coupler 100a which couples portions of the input signal to delay line 140 and to main amplifier 110. Main amplifier 110 produces the amplified output whose signal spectrum is illustrated in FIG. 1C. The additional spectral components shown in FIG. 1C as compared with FIG. 1B are the intermodulation products generated due to nonlinearities in main amplifier 110. A portion of the amplified output signal spectrum shown in FIG. 1C is coupled to summer 150 by coupler 100b. Delay line 140 delays the coupled portion of the input signal with respect to the output of main amplifier 110 producing a delayed signal such that the two signals reach summer 150 at approximately the same time. The output of summer 150 is an error signal which is coupled to auxiliary amplifier 160. Auxiliary amplifier 160 adjusts the amplitude of the error signal producing an error correction signal illustrated in FIG. 1D. The error correction signal should be matched in amplitude to the intermodulation products generated by main amplifier 110, but reversed in phase. The resultant vector cancellation of the intermodulation products is performed in coupler 100c where the error correction signal is subtracted from the amplified input signal. For the output signal illustrated in FIG. 1E to have intermodulation products which are greater than -60 dB down from the carrier frequencies, the vector cancellation must be performed with a high degree of accuracy. Typically this requires that the error correction signal be maintained with greater than 0.5 degrees phase accuracy and 0.1 dB amplitude accuracy which is difficult to achieve in production. The feed-forward technique can be used in an MCPA to effectively suppress intermodulation products but at the cost of low power efficiency and a high demand on complexity and component cost. In particular, high power MCPAs are difficult to master in production.
Accordingly, it would be advantageous to construct a phased array antenna which uses MCPAs in which intermodulation products can be managed without having to resort to expensive and power inefficient feed-forward cancellation techniques.